Finite element analysis of forces of plane cutting of cortical bone

Bone cutting is an essential part of orthopaedic surgery when bone is fractured or damaged by a disease and is used for pins insertion and plates fixation. A finite element model of bone cutting is developed and compared with experimental results. The model allows the interaction between the bone and cutting tool to be studied, hence enabling the evaluation and optimization of the cutting procedure. Results of finite element simulations are obtained for the cutting force as a function of cutting parameters. A strong dependence of cutting parameters on the cutting force was found and described in this paper.     

A study of computational mechanics of 3D spacer fabric: factors affecting its compression deformation

Mechanical properties of a 3D spacer fabric are determined both by mechanical properties of its components and its unique structure. However, at present, there is no well-received method to describe the mechanical performance of a 3D spacer fabric. In this article, numerical methods are used to analyze a compression mechanism of a typical 3D spacer fabric, as an example, based on features of its structure. Micro X-ray computed tomography was used to determine a precise geometry model of the studied fabric. To give a deeper understanding of the effects of the fabric’s structure on its mechanical properties, the finite element (FE) method was used to simulate the compression process of the fabric; seven FE models were developed to simulate its compression behavior. Based on results obtained with these models, the compression mechanism of the 3D spacer fabric is summarized and the factors affecting it are determined. The same method of analysis can be extended to other studies of 3D fabrics.     

An effective thermal property framework for softwood in parametric design fires: Comparison of the Eurocode 5 parametric charring approach and advanced calculation models

Timber, like other structural materials such as concrete and steel, has its own Eurocode (Eurocode 5 part 1.2) for the structural fire design of buildings. However unlike other fire parts of the Eurocodes it is not widely adopted due to its inherent limitations. With the exception of a single Annex, the timber Eurocode (EN 1995-1-2) is only applicable to standard fire exposure. Annex A gives guidance on the charring rates of initially un-protected timber members in parametric fires, however in the UK the use of the Annex is prohibited by the national Annex to the code.The concrete and steel industries have undoubtedly benefited from performance based design whereby the structural fire design strategy is centred on a design fire (typically a parametric fire), which is more credible than the standard fire curve. Such an approach has resulted in more flexible, innovative buildings which have been designed based upon fundamental structural mechanics at elevated temperature, using advanced numerical models. At present however the same principals cannot be applied to the advanced fire design of timber buildings due to current limitations in the timber Eurocode. Where advanced calculation procedures are considered by the code (Annex B), much like many of the methods contained therein, the procedures are only applicable to standard fire exposure.The scope of applicability of the code stems from a fundamental problem regarding a lack of understanding of the heat transfer characteristics of timber in natural fires. The thermo-physical properties contained in the code are ‘effective’ properties. This essentially means that they are calibrated against test results to account for a lack of understanding regarding mass transfer, cracking and ablation both within the timber and char layer. Such calibrations have only been performed on timber members exposed to standard furnace conditions.To attempt to overcome this barrier and extend the scope of thermo-physical properties in the code a study has been undertaken to establish how the conductivity properties of the char layer influence the depth of char in parametric fires. Through calibration of an effective conductivity of the char layer against the parametric charring method contained in Annex A of EN 1995-1-2, it has been possible to establish a relationship between ‘heating rate’ and the effective conductivity of the char layer, in the heating phase of parametric fires. The modified conductivity model is shown to be applicable to a range of densities and moisture contents of timber and also variations in heating rate and fire load density. The latter is a direct result of the method used in the adaptation of the properties. The modified model is objectively critiqued and proposed further work is discussed in detail. The applicability of the modified model in the cooling phase of fires is also discussed.     

Large scale natural fire tests on protected engineered timber floor systems

As part of an ongoing research project to investigate the performance in fire of specific types of innovative construction products and techniques (ICPT), BRE Global have carried out large-scale fire tests to determine the response of different floor systems to a realistic fire scenario. The principal objective was to determine the mode of failure of different floor systems to provide information to key stakeholders (particularly the Fire and Rescue Service), which can be taken into account in the dynamic risk assessments that underpin fire fighting operations. This paper presents the results and observations from those fire tests for three floor systems: (i) solid timber floor joists, (ii) I-section floor beams with solid timber top and bottom flanges and an oriented strand board (OSB) web, and (iii) a timber truss incorporating solid timber upper and lower chord members and a pressed steel web member. These reflect the two most common types of engineered floor systems used in the UK and allow for direct comparison with a more “traditional” form of construction.     

Nonlinear Compression Behavior of Warp-Knitted Spacer Fabric: Effect of Sandwich Structure

Compressibility of warp-knitted spacer fabrics is one of their important mechanical properties with regard to many special applications such as body protection, cushion and mattresses. Due to specific structural features of the fabric and a non-linear mechanical behavior of monofilaments, the compression properties of this kind of fabrics are very complicated. Although several studies have been performed to investigate their compression behavior, its mechanism has not well been understood yet. This work is concerned with a study of compression mechanism of a selected warp-knitted spacer fabric with a given sandwich structure. Both experimental and numerical methods are used to study the effect of the material's structure on the overall compression mechanism. Compression tests are conducted to obtain force-displacement relationships of the fabric. A micro-computed tomography system is used to analyze specimens under different levels of compression displacement to investigate the change in material's structure during the compression process. At the same time, finite element models are developed separately to simulate the initial geometric structure and the compression behavior of the fabric. Three finite element models based on beam elements are firstly developed to simulate the effect of manufacturing process on shapes of monofilaments within the fabric and to determine their morphologies, which are used to assemble a geometry part of the finite element model of the overall fabric. Then the finite-element model is developed using beam and shell elements to describe the compression behavior of the fabric by introducing the effect of its complex microstructure and real non-linear mechanical properties of the monofilaments. A comparison of the obtained experimental and CT data, and results of simulation is carried out, demonstrating a good agreement. With this study, a compression mechanism of the warp-knitted spacer fabric can be better understood.     

Experimental and Numerical Analysis of Damage in Woven GFRP Composites Under Large-deflection Bending

Textile-reinforced composites such as glass fibre-reinforced polymer (GFRP) used in sports products can be exposed to different in-service conditions such as large bending deformation and multiple impacts. Such loading conditions cause high local stresses and strains, which result in multiple modes of damage and fracture in composite laminates due to their inherent heterogeneity and non-trivial microstructure. In this paper, various damage modes in GFRP laminates are studied using experimental material characterisation, non-destructive micro-structural damage evaluation and numerical simulations. Experimental tests are carried out to characterise the behaviour of these materials under large-deflection bending. To obtain in-plane shear properties of laminates, tensile tests are performed using a full-field strain-measurement digital image correlation technique. X-ray micro computed tomography (Micro CT) is used to investigate internal material damage modes – delamination and cracking. Two-dimensional finite element (FE) models are implemented in the commercial code Abaqus to study the deformation behaviour and damage in GFRP. In these models, multiple layers of bilinear cohesive-zone elements are employed to study the onset and progression of inter-ply delamination and intra-ply fabric fracture of composite laminate, based on the X-ray Micro CT study. The developed numerical models are capable to simulate these features with their mechanisms as well as subsequent mode coupling observed in tests and Micro CT scanning. The obtained results of simulations are in agreement with experimental data.     

Viscoelasticity of multi-layer textile reinforced polymer composites used in soccer balls

This paper gives details of a comprehensive dynamic mechanical analysis (DMA) material characterisation activity for all constituent layers of two modern-day thermoformed soccer balls. The resulting material data were used to define a series of viscoelastic finite element (FE) models of each ball design which incorporated the through-thickness composite material properties, including an internal latex bladder, woven fabric-based carcass and polymer based outer panels. The developed FE modelling methodology was found to accurately describe the viscoelastic kinetic energy loss characteristics apparent throughout a soccer ball impact at velocities which are typical of those experienced throughout play. The models have been validated by means of experimental impact testing under dynamic loading conditions. It was found that the viscoelastic material properties of the outer panels significantly affected ball impact characteristics, with outer panel materials exhibiting higher levels of viscous damping resulting in higher losses of kinetic energy.